A portable terminal device 101 of the invention having at least two or more cases joinable in a plurality of different joining shapes includes a magnetism detection section 8 being provided in at least one of the two or more cases for detecting a magnetism quantity; a storage section 9 for storing the effect amount produced by magnetism from a magnetism generation source of the portable terminal device 101 in the detected magnetism quantities for each of the plurality of different joining shapes of the portable terminal device 101; a case shape detection section 4 for detecting a joining shape of the portable terminal device 101; a data processing section 3 for correcting the detected magnetism quantity based on the stored effect amount in the detected joining shape; and a position measurement section 2 for measuring an azimuth based on the corrected magnetism quantity.